A multi-chamber container as a medical bag made of a soft film material is itself known, which has a weak seal constructed by welding opposed surfaces of the medical bag at a relatively low temperature in a manner that the medical bag is divided into partitions for independently string different medical liquids into the respective partitions. An outlet port is made as a plastic mold product and is welded at an outer periphery of the medical bag. The outlet port is formed as a cylindrical shape and has an inner space, which has a first end opened to one of the partitions and a second end located outside the medical bag and having a rubber plug. Prior to giving the medical liquids to a patient, a pressing of the medical bag from its outside is done so that the weak seal is separated and opened, resulting in an unification of inner cavity of the medical bag, thereby mixing the two kinds of medical liquids. Therefore, a succeeding piercing of the plug by a needle of an infusion set permits the medical liquid to be desirably administered. This mixing type medical multi-chamber container necessitates that the opening of the weak seal for mixing the medical liquids is done prior to the administration. In other words, a piercing of the rubber plug of the outlet port without opening the weak seal may cause an erroneous operation to occur that only the medical liquid at the partition located adjacent the outlet port is administered. To combat this problem, a solution has heretofore been proposed that the outlet port is provided with a valve member, which cooperates with an expanded displacement of medical bag as generated when the weak seal is opened. See patent document No. 1.
As a different prior art for solving a problem that an infusion operation is possible without opening the weak seal, a solution has also been proposed that a second weak seal of slightly increased welding temperature is provided at a location upstream from the outlet port, which second weak seal is opened after the opening of the weak seal between the partitions. Refer patent documents 2 and 3.    Patent Document No. 1: Japanese Unexamined Patent Publication 2006-87904    Patent Document No. 2: Japanese Unexamined Patent Publication H09-327498    Patent Document No. 3: Japanese Unexamined Patent Publication 2006-507914